The glue and cement art is a mature technology. Many and varied glues and cements have been described and much speculative disclosure has been made about such compositions. In spite of, or perhaps because of the abundance of information available about glues and cements, needs for improved compositions continue to exist. It has become extremely difficult to find the necessary information about suitable cements for specific purposes.
The need for safer cements has become more apparent as industry and the consumer have become more aware of the potential toxic liabilities of various glue and cement solvents, but these cements must effectively bond. In view of this need for increased safety, certain cements have utilized cyclic 5- or 6-membered ring lactam solvents such as N-methyl-2-pyrrolidone.
U.S. Pat. No. 3,404,117 of Uffner describes an adhesive film-forming composition that, when in the dry state, is insoluble in either water or dry cleaning solvents. The adhesive comprises a blend of one of a certain class of fluorinated copolymers and a plasticizer in a solvent. Some of the solvents claimed are N-methyl-2-pyrrolidone and aliphatic alcohols.
G.B. Pat. No. 1,572,481 describes an ". . . adhesive containing polyvinyl alcohol or ethylene/vinyl alcohol copolymer, a crystalline solvent for the polymer and a viscosity-reducing diluent." The viscosity-reducing diluents include polyhydric alcohols.
U.S. Pat. No. 2,616,868 of Heisenberg and Kleine describes liquid compositions for solutions of polymers and copolymers of vinyl chloride which are used to spin threads. In their description of the prior art, Heisenberg and Kleine point out that there are a series of lactones and lactams which, according to the literature, are " . . . suitable as solvents for polymers and copolymers of vinyl compounds."
Fogle, et al, U.S. Pat. No. 4,152,313, describe an adhesive composition of " . . . vinyl acetate-ethylene emulsion and an admixture of toluene and N-lower alkyl substituted pyrrolidone."
Still other adhesives for use in joining plastics, such as plastic pipes or the like, are known and generally comprised of polymers such as poly(acrylonitrile-butadiene-styrene) (ABS) or poly(vinylchloride) (PVC) in a solvent or a blend of solvents such as methyl ethyl ketone, toluene and/or tetrahydrofuran.
Still other adhesives for use in pipes or the like are known which comprise a suitable polymer such as ABS or PVC and a solvent mixture consisting only of lactones or lactams, and in particular, preferred lactams such as N-methyl-2-pyrrolidone (NMP). These formulations perform well in warm weather and at elevated temperatures, but their properties as bonding agents are less acceptable under low temperature conditions.
Generally, the requirements for solvent cements for use in the plastic industry require that certain strength criteria are met and that the solvents be suitable for use without causing undue user health problems. The standards for solvent cements for use in the United States are specified in Bulletin ASTM D2564-80, published by the American Society for Testing Materials, 1916 Race Street, Phila. Pa. The ASTM Specification for "Solvent Cements for Polyvinyl Chloride Plastic Pipe and Fittings" specifies that the solvent cement should have certain minimum resin content and meet certain minimum requirements for viscosity and strength. For example, the specified minimum resin content for cement for PVC pipe is 10% by weight of the solution and the lap shear strength when tested in accordance with the test procedures requires that the joint between two surfaces have a strength of at least 250 psi after a 2 hour curing time, at least 500 psi after a 16 hour curing time, and at least 900 psi after a 72 hour curing time.
Since, generally, these solvent cements are made from flammable liquids, the specification requires that the solvent should be kept away from sources of ignition and, in addition, ventilation should be maintained not only to minimize fire risk, but to minimize breathing of solvent vapors. In addition, one should avoid direct contact of the solvent cement with the user.
One of the preferred adhesive solvents is N-methyl-2-pyrrolidone. While it is flammable, it has a relatively high flashpoint of 204.degree. F. and, according to the manufacturer, it has a low order of oral toxicity and only produces mild topical effects. Because of these characteristics, N-methyl-2-pyrrolidone is one of the better or safer solvents to use in the solvent cements. However, when N-methyl-2-pyrrolidone alone is used in combination with resins such as ABS or PVC a specified by the ASTM Testing Standards, the solvent adhesive may not pass the lap shear strength requirements.